Brother P-touch 65, Руководство по обслуживанию

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3.2.4 Key Contacts Matrix
[ 1 ] Key contacts matrix
On the main PCB is a key contacts matrix that is a set of carbon-printed 43 key contact
patterns. Each contact pattern has a pair of electrodes.
The rubber key pad is made of high-impedance silicon rubber. As shown in Figure 3.2-3,
each key on the rubber key pad consists of a key top, rubber spring, and conductive
paint which functions as a switching element.
If a particular key is pressed, the conductive paint of the key short-circuits the paired
electrodes carbon-printed on the main PCB.
Figure 3.2-4 shows a key scan timing scheme and scanning pulse outputs. Ports P70
through P75 on the CPU issue a group of key scanning pulses. Every scanning pulse is
Low active for 200 µs and is in high impedance while inactive. Ports P60 through P67
act as input ports which receive key status.
The CPU scans the key contacts matrix every 10 ms. If the CPU reads the same data on
an input port two successive times, it interprets the state as the key being pressed; if the
CPU reads the same data six successive times, it interprets the state as the key being
released. The input mode of this keying system is 2-key roll-over and 3-key lockout.
Figure 3.2-3 Detailed Diagram of Keyboard